Fundamental Quantities and Units

A physical quantity that does not depend on any other physical quantity for its own measurement is called a fundamental (or base) quantity. Physics recognises exactly seven such fundamental quantities: length, mass, time, temperature, electric current, luminous intensity, and amount of substance. Every other physical quantity you will ever measure in physics — however complicated — can ultimately be expressed as some combination of just these seven.

The units used to measure the seven fundamental quantities are correspondingly called fundamental units. In the SI system these are: metre (m) for length, kilogram (kg) for mass, second (s) for time, kelvin (K) for temperature, ampere (A) for electric current, candela (cd) for luminous intensity, and mole (mol) for amount of substance.

Until 20 May 2019, most of these seven base units (apart from the second) were defined either by physical artefacts (like the platinum-iridium kilogram cylinder) or by properties of specific substances, which made them, in principle, subject to slow drift or damage. Since 20 May 2019, following a decision of the 26th General Conference on Weights and Measures, all seven SI base units are instead defined by fixing the exact numerical value of a fundamental physical constant — the metre uses the speed of light $c$, the kilogram uses the Planck constant $h$, the second uses the caesium-133 hyperfine transition frequency, the ampere uses the elementary charge $e$, the kelvin uses the Boltzmann constant $k$, the mole uses the Avogadro constant $N_A$, and the candela uses a fixed luminous efficacy constant $K_{cd}$. Be …